Always use UTF-8 in GsonDeserializer

[oleg-kuzych]

Hello,

in this piece of code:

    protected T readValue(byte[] bytes) throws IOException {
        return gson.fromJson(new String(bytes), returnType);
    }

new String(bytes) will use system default Charset, while UTF-8 should be used according to JWT RFC: https://tools.ietf.org/html/rfc7519#section-7.2

So, if JVM was started with "-Dfile.encoding=ISO-8859-1" system property then GsonDeserializer corrupts data during deserialization:

[coveralls]

Coverage remained the same at 100.0% when pulling 690c812 on oleg-kuzych:fix-gson-unicode-deserialization-issue into 403e189 on jwtk:master.